Abstract
A yellow paper lit by a blue light reflects the same neutral light as a blue paper lit by a yellow light. However, they look rather different. Moreover, neither appears as a grey paper lit by a neutral light which reflects the same light. Using multidimensional scaling we analysed the dissimilarity structure produced by these papers.
Three normal trichromatic observers took part in the experiment. The stimulus display consisted of 3 identical sets of 7 Munsell papers (10B5/12, 10B5/8, 10B6/4, N6.5, 2.5Y7/6, 2.5Y8/10 and 2.5Y8/16) illuminated independently by 3 light sources. The lights were adjusted so that the CIE xy-chromaticity coordinates of the light reflected from yellow paper 2.5Y8/16 under blue light, blue paper 10B5/12 under yellow light, and grey paper under neutral light were close: (0.267, 0.316), (0.288, 0.345), and (0.267, 0.351), respectively. Dissimilarities between the Munsell papers for all the illuminations were evaluated by ranking (see further details of the method in Tokunaga, Logvinenko, & Maloney, Visual Neuroscience, 2008, 25, 395–398). In spite of the close proximity between chromaticity coordinates, the yellow paper under blue light and blue paper under yellow light were judged only slightly less dissimilar than these papers under neutral light by the three observers: 97, 92, and 90%. As all of the lights and papers were simultaneously present in the scene, such a colour constancy effect can hardly be accounted for by chromatic adaptation.
A phenomenon similar to the illumination contrast discounting (see Logvinenko & Tokunaga, VSS'09) was also observed in the chromatic domain. Specifically, the same difference in the CIE chromaticity was found to be judged as less dissimilar when it was produced by the paper difference than by the illumination difference. Hence, while both material and lighting colour difference contributed into dissimilarity judgements, the former was more effective.
EPSRC research grant EP/C010353/1 (AL).